1. Field of the Invention
The present invention relates to a hydraulic pressure control valve which controls pressure oil to be given to two different destinations corresponding to a relative angular displacement between the valve and the casing, particularly to a hydraulic pressure control valve which is used in a hydraulic power steering apparatus in order to control pressure oil to be given to two oil chambers of a hydraulic double acting cylinder for steering assisting corresponding to steering operation.
2. Description of Related Art
There is a hydraulic power steering apparatus which generates hydraulic pressure force in a hydraulic double acting cylinder provided in a steering mechanism, corresponding to direction and strength of steering torque applied to a steering wheel and assists force required for steering operation. In such a power steering apparatus, a hydraulic pressure control valve constructed in the middle of a steering shaft is disposed between a hydraulic pump (pressure oil supply source) driven by an engine and an oil tank (discharge port) maintained at a low pressure, and two oil chambers of a hydraulic cylinder (destination). By the operation of hydraulic pressure control valve, pressure oil to the two oil chambers is controlled corresponding to steering operation.
As a hydraulic pressure control valve, whose cross sectional view is shown in FIG. 1, is generally used. In the control valve, there are provided a cylindrical casing 10 having a plurality of oil grooves 11, 11, . . . extending in the axial direction on its inner peripheral surface at specified intervals and a cylindrical valve body 12 axially and rotatably disposed in the casing 10, and having oil the same number of grooves 13, 13 . . . as the oil grooves 11, 11 . . . , extending in the axial direction, on its outer peripheral surface at specified intervals, thereby aforementioned control operation is carried out according to the relative angular displacement around the axis between the both. The casing 10 and the valve body 12 are so arranged that the former oil grooves 11, 11 . . . and the latter oil grooves 13, 13 . . . are stagger-arranged in the peripheral direction. Both of the communicating portions between the oil groove 11 and the oil grooves 13, 13 in the peripheral direction adjacent on both sides of the oil groove 11 function as two throttling units 2, 3 which change the areas in such a manner that one is increased when the other is decreased according to a relative angular displacement.
In the case where the hydraulic pressure control valve is applied to a power steering apparatus, an input shaft connected with a steering wheel and an output shaft connected with a steering mechanism are coupled coaxially through a tortion bar 4, the casing 10 being fixed coaxially on one joint end and the valve body 12 being integrally constructed on the other joint end. When steering torque is applied on the steering wheel, the relative angular displacement responsive to the tortion of the tortion bar 4 is generated between the casing 10 and the valve body 12. And as shown in the drawing, among the oil grooves 11, 11 . . . at the casing 10 side, half of them positioning every other groove are communicating with one oil chamber of a hydraulic cylinder S for assisting steering and the rest of them are communicating with other oil chamber respectively. Among the oil grooves 13, 13 . . . at the valve body 12 side, half of them positioning every other groove are communicating with a discharge side of a hydraulic pump P being a pressure oil supply source and the rest of them are communicating with an oil tank T being a discharge port respectively. Accordingly, both oil chambers of the hydraulic cylinder S being a destination of pressure oil are communicating with the discharge side of the hydraulic pump P or the oil tank T through one throttling unit consisting of the throttles 2, 2 . . . , and are communicating with the oil tank T or the discharge side of the oil pump P through other throttling unit consisting of the throttles 3, 3 . . . .
In the case where the relative angular displacement is generated between the casing 10 and the valve body 12 responsive to steering torque applied to the steering wheel for example, throttle areas of the throttles 2, 2 . . . are increased and throttle areas of the throttles 3, 3 . . . are decreased, when pressure oil introduced from the hydraulic pump P to each oil groove 13 passes through the throttles 2, 3 on both sides thereof to be flown into the adjacent oil grooves 11, 11, the conduction resistance at the throttle 2 becomes smaller than that at the throttle 3. Therefore, between the oil groove 11 communicating with the oil groove 13 through the throttle 2 and the oil groove 11 communicating with the same through the throttle 3, and between both oil chambers of the hydraulic cylinder S respectively communicating with the oil grooves 11, 11, such pressure difference that the former is higher than the latter is generated, the hydraulic cylinder S generating steering assisting force corresponding to the pressure difference.
In addition, in such a hydraulic pressure control valve, the numbers of the oil grooves 11, 11 . . . at the casing 10 side and the oil grooves 13, 13 . . . at the valve body 12 side are apt to be reduced in order to realize a small-sized valve, and as shown in FIG. 1, the one having six oil grooves 11, 11 . . . and six oil grooves 13, 13 . . . and six throttles 2, 2 . . . and six throttles 3, 3 . . . provided therebetween (hereinafter to be called a six-equally spaced valve) has been put into practical use.
By the way, the strength of the force required for steering a vehicle corresponds to the strength of road reaction force acting upon wheels. In the case where road reaction force is large such as low speed running, large force is required for steering operation. On the other hand, in the case where road reaction force is small such as high speed running, steering can be carried out by relatively small force. Accordingly, in a power steering apparatus, it is desired that such steering assisting force as follows is generated particularly in order to heighten the straight-drive stability. In the case where steering torque to be applied to the steering wheel is small, almost no steering assisting force is generated, and equal rigidity to that of hand steering is given to the steering wheel. When steering torque reaches a predetermined strength, it is required that a large steering assisting force is generated thereby force required for steering operation is to be as small as possible. In the middle state of the both, it is required that steering assisting force is gradually increased in proportion to increment of steering torque in order not to generate unnatural steering feeling by the sudden decrease of the force required for steering operation. Accordingly, desirable increment characteristic of steering assisting force in a power steering apparatus is, as shown in "SAE Technical Paper No. 880706, 1988", so called two-step characteristics as shown in FIG. 2, having a proportional gradual increase region between a constant region wherein steering torque is small and sudden increase region wherein steering torque is large. Further desirable characteristic is the one in which increasing rate (inclination of the gradual increase region in FIG. 2) more than a predetermined rate is obtained in the gradual increase region.
The hydraulic cylinder S generates steering assisting force corresponding to aforementioned operation of the hydraulic pressure control valve, and the strength thereof depends upon the reduction of throttle areas generated in the throttles 2, 2 . . . or the throttles 3, 3 . . . responsive to the relative angular displacement between the casing 10 and the valve body 12. In order to obtain aforesaid two-step characteristics, it is necessary to obtain the decreasing mode of throttle areas which gradually decreases until the relative angular displacement reaches a predetermined value and suddenly decreases when it reaches the predetermined value, in the throttles 2, 2 . . . and the throttles 3, 3 . . . . In order to realize aforesaid decreasing mode, such a hydraulic pressure control valve as represented by U.S. Pat. No. 3,591,136 is well known wherein a notch portion 20 is provided at the corner formed between the inner periphery of the casing 10 and the side wall of the oil groove 11 and having throttles 2 and 3, which consists of, as show in FIG. 3, a first portion 21 crossing the side wall of the oil groove 11 at substantially a right angle and almost in parallel with the inner peripheral surface of the casing 10, and of a second portion 22 crossing the projected end of the portion 21 and the inner peripheral surface of the casing 10 at substantially a right angle to connect them. In addition, FIG. 3 is a view where the peripheral surfaces of the casing 10 and the valve body 12 are unfolded on the straight line. That is to say, in the throttles 2 and 3 provided with the notch 20, until the corner of the valve body 12 side facing the notch portion 20 substantially agrees with the projected end of the first portion 21 according to the increase of the relative angular displacement, constant throttle areas depending only upon the depth d in the radial direction of the notch 20, can be obtained thereby, also by the proportional reduction of the throttle areas in the other throttles 2 and 3 in which the notch 20 is not provided, aforementioned decreasing mode can be obtained. In addition, the notch 20 can be provided at the corner of the valve body 12 side facing the corner of the casing 10 side, however, it is extremely difficult to form the notch 20 at the valve body 12 side. Generally, the notch 20 is formed by cutting the corresponding corner of the casing 10 by polishing after broaching the inner peripheral surface of the casing 10.
Now, as aforementioned equally-spaced six valves to obtain two-step characteristic, there is the one disclosed in Japanese Patent Application Laid-Open No. 61-94870, 1986. The notches are provided at three throttles 2, 2, 2 and the throttles 3, 3, 3 respectively positioning at both sides of either three oil grooves 13, 13, 13 communicating with the discharge side of the hydraulic pump P or those communicating with the oil tank T among six oil grooves 13, 13 . . . at the valve body 12 side in FIG. 1.
In the two-step characteristic obtained, as shown in FIG. 2, by the formation of the notch 20, it is desired as aforementioned that an inclination more than the predetermined one is obtained at the gradual increase region. As the inclination depends upon the sum of the depths d in the radial direction of the notch 20 as shown in FIG. 3 and is apt to increase as the sum decreases, it is necessary to make each depth d of each notch 20 as small as possible in order to obtain the desired two-step characteristic. On the other hand, in forming the notch 20 carried out by aforementioned procedure, a processing error of about 0.02 mm in broaching and that of about 0.005 mm in polishing the inner peripheral surface of the casing 10 are generated respectively. In addition, generation of a core displacement error of about 0.01 mm cannot be prevented during broaching and polishing. The lower limit value of the depth d should be 0.04 mm to secure some degree of oil passage even when these deviations are piled up in the same direction.
However, as disclosed in aforementioned Japanese Patent Application Laid-Open No. 61-94870, 1986, in the case where notches 20 are provided at three throttles 2, 2 . . . and three throttles 3, 3 . . . , even when the respective depths d of the notches are set to the lower limit value 0.04 mm, there has been a problem that the inclination in the gradual increase region of steering assisting force is too small in a power steering apparatus in which the hydraulic pressure control valve is used, thereby steering feeling is worsened by sudden decrease of the operation force for steering wheel in a transit region from gradual increase region to sudden increase region.
The applicants of the present invention has proposed in U.S. Pat. No. 4,924,910 a hydraulic pressure control valve in which communication of the oil tank T with the two oil grooves 13, 13 facing to each other among the four oil grooves 13, 13 . . . to be communicated originally with the oil tank T at the valve body 12 side, is shut off and at the same time such notches 20 as mentioned above are formed at the throttles 2 and 3 on both sides of the shut-off portion, as the one to reduce flowing noises generated during the operation of an eight-equally spaced valve provided with two throttling units having eight throttles. In this hydraulic pressure control valve, as the notches 20 are provided only on two throttles 2, 2 and the throttles 3, 3, a sufficient inclination in gradual increase region can be obtained to satisfy steering feeling in the case where the depths d of the notches are set to the lower limit value 0.04 mm. On the contrary, in the case where this construction is adopted in the manner that six valves are equally spaced, communication of one or two of three oil grooves 13, 13, 13 communicating with the oil tank T should be shut off therewith, thereby pressure imbalance is brought about in the radial direction of the valve body 12. In the case where the imbalance is larger due to a large steering wheel operation, the relative rotation between the casing 10 and the valve body 12 is obstructed and the hydraulic pressure control valve can't operate normally.